Sheet stackers which collect sheets of paper, such as those produced by a printing press and a cutter and a folder assembly, are commonly employed in the prior art in the printing industry. In those prior art stackers, a stream of sheets is collected on a first conveyor which receives the sheets from a folder or the like and which moves the sheets to a stacker or in a stacked relation and on a second conveyor. In this arrangement, the stream of sheets is stripped off the first conveyor and collected in the stack on the second conveyor in an upstanding position. In that arrangement, it is important that the conveyor supporting the stack of sheets move at a critical speed which accommodates the growth of the stack. That speed depends upon the speed of the stream of sheets coming into the stack and also upon the thickness of the stream of sheets, both factors which bear upon the rate of growth of the stack itself. That is, if the stream of sheets is moving relatively fast, then of course the stack will be formed relatively fast; and if the stream of sheets is relatively thick, then the sheets which are stood on edge in the collected stack will cause the stack to be formed relatively fast also.
In summary, the prior art stackers have the two variables of the speed and thickness of the incoming stream of sheets, and those variables determine the rate of growth of the collected stack of sheets. The prior art has accommodated these variables by a manual speed control for the conveyor on which the stack of sheets is being collected and formed. Such manual control can commonly consist of adjusting the pitch of a pulley which forms a part of the conveyor for the collected stack of sheets. However, in that prior art arrangement, it requires that the operator constantly monitor the stacker so that he can make the necessary manual adjustments to get the best speed for the conveyor on which the stack is being formed. One prior art example of that type of manual control is shown in U.S. Pat. No. 2,933,313 wherein the relative speed between two conveyors supporting a stream of sheets is controlled by the manual means of varying the effective pitch of pulleys which are operatively associated with the said two conveyors. The prior art cited also discloses a stack conveyor speed control which relies upon the expansion and contraction of one of the conveyor pulleys or rollers on which the conveyor belt is trained, all to manually adjust the speed of the stack conveyor belt.
For further background and expose of the prior art in stackers of the nature of this invention, reference is made to U.S. Pat. No. 2,884,243 wherein there is a first conveyor for supporting the stream of sheets and a second conveyor for supporting the stack of sheets received from the stream and moving the stack away from the stream. In both instances of the cited prior art, the present invention distinguishes thereover in that it provides an automatic system and method for controlling the speed of the stack conveyor, and that control is made in accordance with the speed and thickness of the incoming stream of sheets, and those two factors can be and generally are variable, and thus there is an automatic method for sensing those two variables and driving the stack conveyor in accordance with those variables so that the stack moves at the appropriate rate of speed.
Other features and advantages and improvements upon the prior art will be apparent to one skilled in the art on reading the following description in light of the accompanying drawings.